This invention relates to optical switches and, more particularly, to an electrically controlled interferometric spatial switch.
An essential component in most optical processing systems is an efficient light switch. Key characteristics of a light switch used in an optical processing system, in which the processed light must commonly pass through many switches, are the amount of optical loss or attenuation that the light beam experiences in passing through the switch and the on-off response time of the switch. A commonly used light switch is a lithium niobate, integrated-optics switch that provides relatively fast on-off response time but typically also has about 3 dB light loss per switch. Thus, for example, if a light beam is passed through only seven such switches in an optical system, it will be attenuated by 21 dB, i.e., the light intensity of the output beam drops to 1/128th of the intensity of the input beam.
One efficient, low loss light switch may typically include a polarizing beam splitter (PBS) and a liquid crystal pixel array to selectively control the polarization of light beams entering the PBS. One example of optical switching using a PBS and a liquid crystal array is disclosed in U.S. Pat. No. 5,117,239 of N. Riza, issued 26 May 1992 and which is assigned to the assignee of the present invention and incorporated herein by reference. As efficient as this switch is, its use is limited to polarized light. Further, the operation of this switch is not based on the principle of optical interference.
Polarization-independent types of beam splitter switches have been suggested that enable an unpolarized light beam to be selectively directed along a selected path. One example of such a polarization-independent switch is described by Wagner and Cheng in "Electrically Controlled Optical Switch for Multimode Fiber Applications," Applied Optics, Vol. 19, No. 17, September 1980, pp 2921-2925. In optical systems, use of polarization-independent switches can be advantageous as there is no reduction of light beam intensity (as may occur if a polarizer is used to polarize light to be used in a polarization-dependent system) and connections between blocks of the optical system can be made with optical fibers that do not require polarization-maintaining fibers. Although the switch suggested by Wagner et al is polarization-independent, it requires high quality polarization-based optical components, such as PBSs, which add at least twice to reduction of light beam intensity as well as to the complexity and cost of the switch, being that high quality polarization-based optical components are, in general, more elaborate than non-polarization based optical components. Here again, Wagner et al do not suggest either an interferometric switch, i.e., a switch based on the principle of optical interference between substantially coherent light beams, or the use of non-polarization based optical components.
A high-speed nematic liquid crystal modulator using the so called high voltage transient nematic effect is described by S. T. Wu in "Nematic Liquid Crystal Modulator with Response Time Less than 100 .mu.s at Room Temperature", Applied Physics Letters, Vol. 57, No. 10, September 1990, pp 2921-2925. However, Wu does not suggest how to use the high voltage transient nematic effect to make an interferometric optical switch based on a high-speed nematic liquid crystal cell. Additionally, Wu suggests neither an interferometric optical switch based on a polarization-independent nematic liquid crystal cell nor a cascading arrangement of nematic liquid crystal cells to further reduce the on-off response time of the interferometric optical switch. A need thus exists for a compact low-loss light switch which can operate either on polarized or unpolarized light using non-polarizing optical components. Further, it is desirable to provide a light switch whose on-off response time is comparable or superior to that of currently available fast-response switches (e.g., 100 .mu.sec).
Accordingly, one object of the invention is to provide an interferometric light switch than can operate either on polarized or unpolarized light.
Another object of the invention is to provide an interferometric switch having a relatively fast on-off response time.
It is yet another object of the invention to provide a compact, low-loss interferometric light switch which uses non-polarizing optical components.